Out of This World – Abell 520

Now would be a good time to queue the theremin, that weird little instrument responsible for the eerie soundtrack in most old sci-fi movies. We are leaving Earth’s surface to explore a mind (and light) bending phenomenon. NASA astronomers are observing a clump of dark matter left in the wake of a galactic collision. 2.4 billion light-years away sits this amalgamation of an immense group of galaxies labeled Abell 520.

Dark matter itself is not detectable by any known instruments but is found by the effect it has on its surroundings. Light from galaxies is bent and distorted through gravitational lensing allowing astronomers to infer that dark matter is present. While this substance makes up the majority of the universe’s mass, little is quantitatively known about it. Up until this recent study, it was believed that objects are anchored to dark matter so that when galaxies collide, they then stick close to the dark matter. This was observed in other massive colliding galaxies such as the Bullet Cluster and was considered the model to follow. Abell 520 defies this assumption as instead of the galaxies coalescing in the center of the dark matter, the Hubble telescope detected no luminous bodies in the core.

Above is a composite image of the Abell 520 collision created from a number of different sensors. The orange color is starlight from galaxies, the green area is hot gas and most of the mass is blue. The area where the blue and green blend is the core of the dark matter. It is here that hot gas resides and very few luminous bodies are detected. The base image depicting the luminous bodies was acquired by Hubble. (Image Source: NASA)

There are few examples of colliding galaxies that have been mapped by astronomers – the Bullet Cluster was the best example and showed a very different scenario. In this case, luminous galaxies were anchored to the dark matter after the collision. With so few examples readily available, astronomers have come up with a few explanations as to why these phenomenons differ greatly. One possibility is that some dark matter is stickier than others so that when galaxies collide, the dark matter from each will interact and slow down, while other dark matter continues to move normally. Another possible scenario is that the Abell 520 cluster consists of more than just two galaxies colliding hence different dark matter interactions could occur. There is also the possibility that Hubble was not able to detect the galaxies at the core of the dark matter, which would mean that the colliding galaxies have less stars then most normal galaxies.

The astronomers plan to create a computer simulation of the collision to hopefully find some answers to the questions they uncovered. So we are back to where we started with mysterious dark matter that makes up the majority of the universe that we hurl through. This new study has strong implications, making us aware that the universe is still a vast mystery; and until the Starship Enterprise can explore these unknown worlds, that we are very much in the dark.